The use of polyvalent metal chelates in general and of iron chelates in particular is well known for the removal of H.sub.2 S from sour gas or liquid streams. For example, R. T. Jernigan in U.S. Pat. Nos. 4,414,817, 4,468,929 and 4,528,817 discloses the treatment of geothermal steam by condensing it with an aqueous solution of a ferric chelate such as that of hydroxyethylethylenediaminetriacetic acid (HEDTA) or ethylenediaminetetraacetic acid (EDTA). The hydrogen sulfide is converted to elemental sulfur and the ferric chelate is reduced to ferrous oxidation state. The ferrous chelate is subsequently oxidized back to the ferric oxidation state and recycled. Z. Diaz in U.S. Pat. Nos. 4,400,368 and 4,518,576 discloses similar processes for treating sour gas streams, while Jeffrey et al., further expand such applications to drilling fluids and muds in U.S. Pat. No. 4,756,836.
More recently, U.S. Pat. No. 4,816,238 calls for the preferred treatment of sour gases containing H.sub.2 S with aqueous alkaline solutions of polyvalent metal chelates at a pH of about 7 to 10. The ferric complex of HEDTA is much more stable than that of EDTA at pH values above 8. Under these conditions, the ferric complex of HEDTA is usually required to assure that the iron remains in solution or does not react with sulfide to form FeS.
Aminopolycarboxylic acids such as HEDTA and EDTA are commercially produced as the sodium salts. If the acid form is desired, further processing is necessary. To prepare EDTA acid, about a 40 weight percent aqueous solution of tetrasodium EDTA is pH adjusted to about 2.2 with mineral acid. Since EDTA is relatively insoluble in water, it is readily obtained by simple filtration of the precipitate. The HEDTA acid, on the other hand, remains soluble in water under acid conditions and is very difficult to isolate. As a result, HEDTA acid is not readily available on a commercial scale.
Aqueous solutions of iron chelates of aminopolycarboxylic acids are conveniently prepared using iron oxide (Fe.sub.3 O.sub.4) and the aminopolycarboxylic acid; see U.S. Pat. Nos. 3,767,689 and 4,438,040. With only the trisodium salt of HEDTA being readily available, however, only water soluble iron salts are suitable as the source of iron. Typically, ferric HEDTA is prepared using iron nitrate and an aqueous solution of trisodium HEDTA. Like many other nitrate containing mixtures, however, the resulting chelate prepared from iron nitrate may be thermally unstable. Furthermore, neither the iron halides nor iron sulfate are preferred ever iron nitrate because of inherent problems associated with their counterions. The halides from the iron halides cause unacceptable corrosion problems while the sulfate from iron sulfate exhibits stability problems in concentrated solutions where sodium sulfate decahydrate is prone to precipitate. Thus, it is highly desirable to have a stable aqueous iron chelate for use in H.sub.2 S abatement in alkaline systems and to have a convenient method to manufacture such a chelate.